Optical transmission device and method for driving laser diode

ABSTRACT

An optical transmission device obtains driving control data corresponding to a temperature detected by a temperature detection circuit ( 112 ) from memory means ( 173 ), controls a driving current to be supplied to a laser diode ( 100 ) based on the driving control data, and measures a driving current to be actually supplied to the laser diode whose emission power is held constant by an automatic optical output control circuit ( 115, 113 ). Further, when the difference between the measured driving current and a driving current determined by the driving control data corresponding to the detected temperature at that time exceeds an allowable range, the optical transmission device updates the driving control data related to the corresponding temperature, on the memory means so that the difference between the driving currents is defined as each of increases in bias current and modulating current.

This is a divisional application of U.S. Ser. No. 09/462,992, filed Jan.18, 2000 filed as a 371 of PCT/JP97/03260 filed Sep. 16, 1997.

TECHNICAL FIELD

The present invention relates to an optical transmission device having alaser diode and a method for driving the laser diode, and specificallyto a technique for optimizing a driving current of a laser diode evenwith respect to deterioration in characteristic of the laser diode dueto secular changes, e.g., a technique effective for application to adigital optical communication system.

BACKGROUND ART

A laser diode emits light when a driving current thereof exceeds anoscillating threshold current (called simply threshold current). Theintensity or power of emission thereof is proportional to a modulatingcurrent corresponding to the current exceeding the threshold current. Inorder to make fast the speed of response of an emitting operation of thelaser diode, the threshold current or a neighboring current thereof iscaused to flow at all times, and the modulating current is allowed toflow as a pulse current corresponding to a data signal in formsuperimposed on the bias current. As a result, an optical pulse can begenerated.

The stable execution of optical communications needs to hold theintensity of light at its emission constant. At this time, the emissioncharacteristic of the laser diode greatly depends on the temperature.Namely, the threshold current becomes great as the temperature rises.Further, the modulating current necessary to obtain predeterminedemission power becomes also great as the temperature rises. The emissioncharacteristic of the laser diode is deteriorated due to secularchanges, and the threshold current becomes great as its using periodbecomes long. The modulating current necessary to obtain thepredetermined emission power is also made great. Further, thecharacteristic change corresponding to the temperature and secularchanges differs between the threshold current and the modulatingcurrent.

In order to cope with such a characteristic change, an auto powercontrol technique has heretofore been adopted wherein the mean level ofemission power of a laser diode is detected from a current flowing in aphotodiode provided so as to be opposed to the laser diode and a biascurrent is increased by the amount equivalent to a reduction in thedetected level, whereby fixed emission power can be obtained. JapanesePatent Application Laid-Open No. Hei 8-204268 is known as an example ofa reference in which the present technique has been described.

In the above-described prior art, however, only the bias current ischanged and no modulating current is controlled. Therefore, if the biascurrent exceeds the threshold current, then a quenching failure occurs.If the bias current is excessively smaller than the threshold current inreverse, then a quenching delay occurs. In a word, the prior art merelycontrols the sum of the bias current and the modulating current withrespect to changes in threshold current of the laser diode andmodulating current for obtaining fixed emission power of the laser diodedue to a change in temperature and secular changes.

On the other hand, a technique for controlling a driving current of alaser diode while paying attention to both changes in threshold currentcorresponding to the temperature and modulating current for obtainingfixed emission power has been described in Japanese Patent ApplicationLaid-Open No. Hei 6-61555. Namely, current ratio control data definingthe ratio between the optimum bias current and modulating current foreach operating temperature of the laser diode is prepared in a ROM orthe like. The current ratio control data is read from the ROM inaccordance with the result of detection of the operating temperatures ofthe laser diode, and reference is made to the driving current of thelaser diode subjected to auto power control, whereby the bias currentand modulating current are determined according to the current ratiocontrol data with respect to the driving current.

However, the aforementioned prior art has no taken into considerationthe deterioration in characteristic of the laser diode due to thesecular changes. According to the discussions of the present inventors,it was revealed that if a distinction between whether an increase indriving current by the auto power control results from a change inambient temperature and whether it results from deterioration incharacteristic of the laser diode due to the secular changes would notbe done, it was difficult to optimize both the bias current and themodulating current.

Further, the wavelength of output light also varies with thedeterioration of the laser diode. This occurs because the laser diode isdeteriorated and the driving current for obtaining the required opticaloutput increases to thereby increase the temperature of an active layerof the laser diode and shift the output wavelength to the long-waveside. If the temperature of the active layer is lowered, then thewavelength of the optical output is shifted to the short-wave side. Suchchanges in wavelength cause a recognition error of a transmission signalin, for example, a system for performing wavelength-divisionmultiplexing transmission.

An object of the present invention is to provide an optical transmissiondevice capable of improving the reliability of light-based informationtransmission.

Another object of the present invention is to provide an opticaltransmission device which reduces a quenching failure and an emissiondelay to the minimum with respect to deterioration in characteristic ofa laser diode due to a change in ambient temperature and secular changesto thereby make it possible to hold an optical output constant.

A further object of the present invention is to provide a method ofreducing a quenching failure and an emission delay to the minimum withrespect to deterioration in characteristic of a laser diode due to achange in ambient temperature and secular changes to thereby drive thelaser diode.

A still further object of the present invention is to provide an opticaltransmission device capable of relaxing a change in the wavelength of anemission output.

DISCLOSURE OF THE INVENTION

An optical transmission device according to the present inventioncomprises a laser diode, a current supply circuit for supplying a biascurrent and a modulating current superimposed on the bias current to thelaser diode as driving currents, an automatic optical output controlcircuit for supplementing the shortage of the driving currents so thatemission power of the laser diode is held constant, a temperaturedetection circuit for detecting an ambient temperature of the laserdiode, memory means storing driving control data for determining amodulating current and a bias current necessary to obtain predeterminedemission power therein for each predetermined temperature, and controlmeans for obtaining driving control data corresponding to thetemperature detected by the temperature detection circuit from thememory means, controlling each driving current to be supplied from thecurrent supply circuit to the laser diode, based on the obtained drivingcontrol data, measuring each driving current actually supplied to thelaser diode whose emission power is held constant by the automaticoptical output control circuit, detecting whether the difference betweenthe measured driving current and a driving current determined accordingto the driving control data corresponding to the detected temperature atthat time exceeds an allowable range, and updating the driving controldata related to the corresponding temperature, on the memory means sothat the difference between the driving currents is defined as each ofincreases in bias current and modulating current.

The range allowable for the increase in driving current is a range inwhich a quenching failure and an emission delay substantially show noproblem when automatic optical output control is effected on a drivingcurrent formed by driving control data at a given temperature, forexample. This can be defined as a current corresponding to about a few %of the driving current, for example.

According to the above-described means, the driving control datacorresponding to the detected temperature is used to control the drivingcurrent of the laser diode. Upon determination of the deterioration ofthe laser diode, the ambient temperature of the laser diode is furthermeasured and a decision is made as to whether the difference between adriving current defined by driving control data corresponding to thenewly measured temperature and an actual driving current formed by theautomatic optical output control exceeds an allowable value. When thedifference is found to exceed the allowable value, it is determined thatthe deterioration of the laser diode has been advanced. Thus, adistinction between whether the increase in driving current due to theautomatic optical output control results from the deterioration of thelaser diode and whether it results from a variation in ambienttemperature is reliably done. The driving control data corresponding tothe corresponding temperature is updated based on the difference betweenthe driving currents used to determine the deterioration of the laserdiode. After the renewal of the driving control data, the drivingcurrent for the laser diode is controlled using the correspondingupdated driving control data. Thus, a quenching failure and an emissiondelay are limited to the minimum with respect to both the change inambient temperature and the characteristic deterioration of the laserdiode due to secular changes, whereby an optical output can be heldconstant.

Once the driving control data is updated, the driving currents under thecorresponding temperature are set as a bias current and a modulatingcurrent determined by the updated driving control data. The subsequentdetection of the deterioration in the laser diode is carried out bydetermining whether the difference between the driving currentdetermined by the corresponding updated driving control data and thedriving current subjected to the automatic optical output controlexceeds the allowable value. Thus, when the progress of thedeterioration of the laser diode is detected, the previously-storeddriving control data is renewed into driving control data includinginformation for defining the newly-acquired amount of correction.Thereafter, the driving current of the laser diode is determinedaccording to the updated correction driving control data.

The driving control data comprises initial data for initiallydetermining the bias current and the modulating current for eachpredetermined temperature, and correction data subsequently added to theinitial data. At this time, the correction data can be set as data fordefining the difference between the driving currents as each of theincreases in bias current and modulating current. Described morespecifically, the control means can include data about the differencebetween the driving currents and a value obtained by increasing thedifference between the driving currents by a factor of a constant ratiosmaller than 1 in the correction data as data about an increase in biascurrent. At this time, the control means determines a bias current bythe sum of the initial bias current data included in the initial dataand the data about the increase in bias current included in thecorrection data when the driving current for the laser diode isdetermined based on the initial data and the correction data, and addsthe initial modulating current data included in the initial data to thedifference between the data about the difference between the drivingcurrents and the data about the increase in bias current respectivelyincluded in the correction data, thereby making it possible to determinea modulating current.

In order to relax a change in wavelength incident to a rise in thetemperature of the laser diode, a cooling device capable of selectivelycooling the laser diode is further provided. The control means iscapable of lowering an ambient temperature of the laser diode by apredetermined temperature by the cooling device each time the differencebetween the measured driving currents reaches a predetermined value withrespect to a driving current defined by initial driving control data ora driving current defined by correction driving control data and theinitial driving control data.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing one example of an optical transmissiondevice according to the present invention with an optical transceiver asthe core;

FIG. 2 is a block diagram schematically illustrating the entire opticaltransmission device constructed as an interface board.

FIG. 3 is a circuit diagram depicting one detailed example of theoptical transceiver;

FIG. 4 is a circuit diagram showing one example of a switching controlcircuit for allowing a modulating current to flow in a diode in pulseform;

FIG. 5 is an explanatory view illustrating one example of a drivingcontrol data table;

FIG. 6 is an explanatory view depicting the relationship between adriving current and emission power of an LD;

FIG. 7 is an explanatory view showing a temperature characteristic ofthe LD;

FIG. 8 is an explanatory view illustrating deterioration-liferelationship between a driving current Id and emission power Pf of theLD under a fixed temperature environment;

FIG. 9 is a flowchart depicting a procedure for generating data fordriving control of the LD using the driving control data table andcorrection driving control data therefor;

FIG. 10 is an explanatory view showing, as an example, the relationshipbetween an increase δId in driving current and an increase δIb in biascurrent by auto power control;

FIG. 11 is a flowchart illustrating the transition of the drivingcontrol of the LD according to a progress in characteristicdeterioration of the LD; and

FIG. 12 is a flowchart showing one detailed example of a coolingprocess.

BEST MODE FOR CARRYING OUT THE INVENTION

<<Optical transmission device>>

One embodiment of an optical transmission device is illustrated in FIG.1 with an optical transceiver as the core. An optical transceiver IT andan optical receiver 1R are typically illustrated in the opticaltransmission device 1 shown in the same drawing. Although not restrictedin particular, the optical transceiver 1T is provided with a laser diodemodule 10, a driver circuit 11, an input circuit 12, and a microcomputer17 respectively individually brought into semiconductor integratedcircuits. The optical receiver 1R includes a pin photodiode 13, apro-amplifier 14, a main amplifier 15 and an output circuit 16respectively individually brought into semiconductor integratedcircuits.

The laser diode module 10 has a laser diode (also described as LD) 100and a photodiode (also described as, PD) 101 for monitoring. An opticaloutput of the laser diode 100 is outputted to an optical output terminalOPOUT. The pin photodiode 13 receives a light or lightwave signal froman optical input terminal OPIN. The input circuit 12 is connected to adata input terminal DTIN and a clock input terminal CLIN, whereas theoutput circuit 16 is connected to a data output terminal DTOUT and aclock output terminal CLOUT.

The input circuit 12 has a buffer memory 120 and an input buffer 121such as a D type flip flop (F/F) or the like. The buffer memory 120successively stores data signals sent from the data input terminal DTINin synchronism with a clock signal inputted from the terminal CLIN. Thedata stored in the buffer memory 120 is supplied to the input buffer 121in synchronism with the clock signal supplied from the clock inputterminal CLIN, where it is waveform shaped, followed by supply to thedriver circuit 11.

The driver circuit 11 has an LD driver 110 and an auto power controller(APC) 111. The LD driver 110 allows a bias current corresponding to athreshold current of the LD100 to flow in the LD100 and selectivelysupplies a modulating current for on/off-controlling the LD100 to theLD100 in response to each data signal supplied from the input buffer120.

The PD101 effects photoelectric conversion on light outputted from theLD100 to form or produce a current corresponding to the intensity orpower of emission of the LD100. The APC111 auxiliarily controls adriving current supplied to the LD100, based on the current flowing inthe PD101 so that the emission power of the LD100 becomes constant. Themicrocomputer 17 performs basic control related to the bias current andmodulating current on the LD driver 110. The details thereof will bedescribed later. The optical output of the LD100 is supplied from theoptical output terminal OPOUT to a transmission line such as an opticalfiber or the like.

The pin photodiode 13 detects the light signal supplied to the opticalinput terminal OPIN from the transmission line and converts ortransforms it to a received signal current. The received signal currentis converted into a voltage signal by the pre-amplifier 14. Theconverted voltage signal is supplied to the main amplifier 15. The mainamplifier 15 amplifies the input voltage signal up to an ECL level. Theoutput circuit 16, which receives the output of the main amplifier 15therein, has a timing extractor 160, a recognizer or identifier 161 andan output buffer 162 like a flip-flop. The timing extractor 160 dividesthe input signal into two systems. Further, the timing extractor 10delays one of them and ANDs it and the other thereof to thereby producea pulse containing a clock component of, for example, 155.52 MHz.

Only the clock component of 155.52 MHz is extracted from the pulse by anunillustrated SAW (Surface Acoustic Wave) filter and limit-amplified toproduce a clock signal. The identifier 161 sufficiently amplifies theinput signal supplied from the main amplifier 15 and waveshapes it intoa signal whose waveform's upper and lower portions are sliced. Theoutput buffer 162 performs waveform shaping (suppression on pulse-widthdistortion) on-the so-sliced signal, using the clock signal. The outputof the output buffer 162 is supplied to the data output terminal DTOUTand the clock signal produced by the timing extractor 160 is supplied tothe clock output terminal CLOUT.

The optical transceiver 1T shown in FIG. 1 is provided with themicrocomputer 17. Although not restricted in particular, themicrocomputer 17 is used even for control of the optical receiver 1R.

Although not restricted in particular, the microcomputer 17 has a CPU(Central Processing Unit) 170, a RAM (Random Access Memory) 171, a ROM(Read Only Memory) 172, a flash memory 173 illustrative of one exampleof an electrically erasable and programmable non-volatile memory device,an input/output (I/O) circuit 174, etc. They are connected to aninternal bus 175. Although not restricted in particular, the ROM172 is amask ROM which stores constant data or the like therein, whereas theRAM171 is defined as a work area or region of the CPU170. Further, theflash memory 173 holds an operating program, driving control data, etc.for the CPU170 therein so that they are programmable.

The microcomputer 17 is a circuit for controlling the opticaltransmission device 1 over its entirety. The driving control data forthe LD100 is stored in the flash memory 173. When the LD100 is driven totransmit light, the CPU170 reads driving control data corresponding to atemperature detected by a temperature sensor 112 to be described laterfrom the flash memory 173 and performs driving control of the LD100 bythe LD driver 110, based on the read data. Namely, a data table (drivingcontrol data table) created based on a temperature characteristic of theLD100 is prepared in the flash memory 173. Thereafter, the CPU170controls a driving current flowing in the LD100 in conformity with thetemperature characteristic of the LD100 according to the optical output,temperature, etc. necessary for the LD100. The contents of control onthe driving current will be described later. In addition to the above,the microcomputer 17 controls the gain of the pre-amplifier 14 byswitching.

The microcomputer 17 is connected to an unillustrated protocolcontroller or the like lying within the optical transmission devicethrough a microcomputer interface terminal (also called micon interfaceterminal) MCIF so as to be supplied with instructions for transmissionand reception control, etc. The micon interface terminal MCIF isconnected to a mode terminal of the microcomputer 17 and a predeterminedport of the input/output circuit.

The microcomputer 17 has, for example, a boot mode in addition to a userprogram mode. When the user program mode is set to the microcomputer 17,the CPU170 executes the operating program stored in the flash memory173. The boot mode is an operation mode for allowing the flash memory173 to be rewritten or programmed directly from outside themicrocomputer 17. When the boot mode is set to the microcomputer 17, theinput/output circuit 174 is brought to a signal input/output statecapable of externally directly programming or rewriting the flash memory173. Namely, when the boot mode is set, a rewriting high voltage, aprogram signal, addresses and data can be transferred to and from theflash memory 173 through the micon interface terminal MCIF. By using theboot mode, the driving control data can initially be written into theflash memory 173 and the operating program for the CPU170 can be writteninto the flash memory 173. It is also possible to rewrite the flashmemory 173. The writing and rewriting of data into the flash memory 173can be carried out even by the user program mode of the microcomputer17. The driving control data written into the flash memory 173 can bereprogrammed under the control of the CPU170.

FIG. 2 is an overall block diagram of the optical transmission device.Although not restricted in particular, the optical transmission deviceshown in the same drawing constitutes one of a large number of interfaceboards which make up of an ATM switch or the like. The opticaltransceiver 1T and the optical receiver 1R are connected to an opticaltrunk network by an optical fiber. A SUNI (Serial User NetworkInterface) 3 is provided at a stage subsequent to the opticaltransceiver 1T and the optical receiver 1R. Deserialization orserial-parallel conversion of data is done at a portion of the SUNI3,which is connected to the optical transceiver 1T, whereas serializationor parallel-serial conversion of data is carried out at a portion of theSUNI3, which is connected to the optical receiver 1R. When the protocolcontroller 4 supports an ATM (Asynchronous Transfer Mode), it performsassembly/de-assembly of each data cell and multiplexing/de-multiplexingthereof. A transmit signal is supplied to the protocol controller 4through a parallel input circuit 6 and a transmitting buffer 5. Areceive signal is supplied from the protocol controller 4 to a paralleloutput circuit 8 through a receiving buffer 7. The parallel inputcircuit 6 and the parallel output circuit 8 are connected to anotherswitch through, for example, an unillustrated interface cable or thelike.

<<Optical transceiver>>

FIG. 3 shows one detailed example of the optical transceiver 1T. The LDdriver 110 has a transistor Tr1 for determining a bias current fedthrough the LD100 and a transistor Tr2 for determining a modulatingcurrent to be supplied to the LD100, as current source transistors.Transistors Tr3 and Tr4 are switching transistors for controlling on andoff of the modulating current fed through the LD100. The transistors Tr1through Tr4 are constructed as npn type bipolar transistors.

The transistors Tr3 and Tr4 are electrically connected in parallel. Acommon emitter thereof is electrically connected to the collector of thetransistor Tr2. The emitter of the transistor Tr2 is coupled to a groundvoltage GND through a resistor R2. The cathode of the LD100 iselectrically connected to the collector of the transistor Tr3. The anodeof the LD100 and the collector of the transistor Tr4 are commonlyconnected to a source voltage Vcc.

As one detailed example of a switching control circuit 114 for thetransistors Tr3 and Tr4 is illustrated in FIG. 4, a series circuit oftransistors Tr5 and Tr6 and a series circuit of transistors Tr7 and Tr8are placed between the source voltage Vcc and the ground voltage GND.The transistors Tr5 through Tr8 are constructed as npn bipolartransistors. The bases of the transistors Tr6 and Tr8 are biased by apredetermined voltage and serve as load resistors for the transistorsTr5 and Tr7. In other words, the series circuit of the transistors Tr5and Tr6 and the series circuit of the transistors Tr7 and Tr8respectively constitute emitter follower circuits. The emitter of thetransistor Tr5 is electrically connected to the base of the transistorTr3, and the emitter of the transistor Tr7 is electrically connected tothe base of the transistor Tr4.

The bases of the transistors Tr5 and Tr7 are supplied with adifferential output produced from a differential output amplifier AMP.When their inputs are reversed, the states of potentials applied to thebases of the transistors Tr3 and Tr4 are inverted. The amplifier AMP issupplied with the output of the selector 121.

When the potential applied to the base of the transistor Tr3 is broughtto a high level, the transistor Tr3 is shifted to a saturated state.When the base of the transistor Tr4 is brought to the high level, thetransistor Tr4 is shifted to the saturated state. The transition of thetransistors Tr3 and Tr4 to the saturated states is complementarilycarried out so that the transistors Tr3 and Tr4 are complementarilyswitched, thus causing a pulse-shaped modulating current to flow in theLD100 through the current source transistor Tr2.

As shown in FIG. 3, the collector of the transistor Tr is electricallyconnected to the collector of the transistor Tr3, whereas the emitterthereof is electrically connected to the ground voltage GND through aresistor R1. The transistor Tr1 allows a bias current equivalent to athreshold current to flow in the LD100 according to the base voltageapplied thereto.

The PD101 is electrically series-connected to a resistor R3 and placedin a backward connected state between the source voltage Vcc and theground voltage GND. The PD101 supplies a current corresponding toemission intensity or power outputted from the LD100.

Referring to FIG. 3, the input/output circuit 174 of the microcomputer17 is illustrated in form divided into a digital-analog converter (D/A)176 for converting a digital signal into an analog signal, ananalog-digital converter (A/D) 177 for converting an analog signal to adigital signal, a timer 179 and another input/output circuit 178. TheD/A176 has three D/A conversion channels DAC1 through DAC3, and theA/D177 has four A/D conversion channels ADC1 though ADC4.

The D/A conversion channel DAC3 has an intrinsic register accessed bythe CPU7 and output a signal for driving a cooling driver 201 to bedescribed later. The D/A conversion channels DAC1 and DAC2 respectivelyhave intrinsic registers accessed by the CPU170. Further, the D/Aconversion channels DAC1 and DAC2 convert values of the correspondingregisters into D/A form and outputs base bias voltages for thetransistors Tr1 and Tr2. Although not restricted in particular, each ofthe D/A conversion channels DAC1, DAC2 and DAC3 converts a 8-bit digitalsignal to an analog signal in a 256-step gradation.

As described above, the modulating current to be fed through thetransistor Tr3 in accordance with on/off-control of the optical outputis determined according to driving control data set to the D/Aconversion channel DAC2 by the CPU170. The bias current to be fedthrough the LD100 is determined according to driving control data set tothe D/A conversion channel DAC1 by the CPU170.

Thus, the CPU170 is capable of individually and arbitrarily controllingthe modulating current and bias current capable of being supplied to theLD100 in accordance with the driving control data set to the D/Aconversion channels DAC1 and DAC2. Thus, the CPU170 sets datacorresponding to the temperature characteristic of the LD100 or the likewith respect to a use condition (use atmospheric temperature) of theoptical transmission module 1 to the D/A conversion channels DAC1 andDAC2, in other words, the CPU170 sets data corresponding to a biascurrent equivalent to the threshold current of the LD100 at a useambient or environmental temperature at that time to the D/A conversionchannel DAC1 and sets data corresponding to a modulating current to beadded to the bias current to the D/A conversion channel DAC2 to obtainnecessary emission intensity or power under the temperature thereof,thereby making it possible to light-produce and drive the LD100 withouta quenching error and an emission delay.

The A/D conversion channels ADC1 through ADC4 are successively assignedto their corresponding inputs of an emitter voltage of the transistorTr1, an emitter voltage of the transistor Tr2, an anode voltage of thePD101 and an output voltage of the temperature sensor 112, and haveregisters inherent in them, for holding the results of A/D conversion ofthe assigned input voltages therein so as to be accessible by theCPU170. Although not restricted in particular, the A/D conversionchannels ADC1 through ADC4 respectively have 10-bit conversion accuracy.

Thus, the CPU170 is capable of monitoring a bias current fed through orflowing in the transistor Tr1, a modulating current flowing in thetransistor Tr2, a current flowing in the PD201, and the output of thetemperature sensor 10 through the A/D converter 177 as needed. Althoughnot restricted in particular, the operation of the CPU170 for monitoringtheir information can be carried out each time the CPU170 receives atimer interrupt given from the timer 179.

The output of the monitor PD101 is made available even to auto powercontrol (automatic optical output control). The APC111 shown in FIG. 1is comprised of, for example, a comparator 113 and an APC controlcircuit 115 shown in FIG. 3. Namely, the comparator 113 receives ananode voltage corresponding to a current fed through the PD101 accordingto actual emission power of the LD100, determines whether the inputvoltage is smaller than a reference potential Vref corresponding topredetermined emission power, superimposes a signal corresponding to theresult of determination over the signal outputted from the D/Aconversion channel DAC1 and supplies it to a base electrode of thetransistor Tr1, and increases or decreases a bias current fed throughthe LD100 through the transistor Tr1. The APC control circuit 115 is acircuit for forming the reference potential Vref, which forms thereference potential Vref, based on the mean value of current fed throughthe PD101 according to the emission power of the LD100 and the meanvalue (mark rate) relative to the input signal of the amplifier AMP atthat time. Although not restricted in particular, the auto power controlis supplementary to bias current control based on the output of the D/Aconversion channel DAC1. Namely, it supplements an error unable to befollowed up by the bias current and modulating current based on theoutput of the D/A176.

The CPU170 can monitor the anode voltage of the PD101 through the A/Dconversion channel ADC3, recognize the actual emission intensity orpower of the LD100, and detect, for example, a state in which the actualemission power is reduced with respect to target emission power. TheCPU170 is capable of monitoring the emitter voltage of the transistorTr1 through the A/D conversion channel ADC1, converting the monitoredvoltage to a current, comparing the value of the converted current and abias current to be fed through the transistor Tr1 via the D/A conversionchannel DAC1, and detecting an abnormal variation in bias current, basedon the difference therebetween. Similarly, the CPU170 is capable ofmonitoring the emitter voltage of the transistor Tr2 through the A/Dconversion channel ADC2, converting the monitored emitter voltage to acurrent, comparing the value of the converted current and a modulatingcurrent to be fed through the transistor Tr2 via the D/A conversionchannel DAC2, and detecting an abnormal variation in modulating current,based on the difference therebetween.

Further, the CPU1710 monitors the emitter voltages of the transistorsTr1 and Tr2 through the A/D conversion channels ADC1 and ADC2respectively and converts the monitored voltages to their correspondingcurrents, thereby making it possible to measure a driving current(corresponding to the sum of the bias current and the modulatingcurrent) fed through the LD100 in practice. The measured current alsoincludes an increase in bias current by the auto power control.Accordingly, the CPU170 is capable of grasping or keeping track of thedifference between the driving current measured in this way and thedriving current to be supplied to each of the transistors Tr1 and Tr2through each of the D/A conversion channels DAC1 and DAC2.

<<Driving control data for LD>>

The driving control data for determining the modulating current and thebias current to be supplied to the LD100 is stored in a driving controldata table. The driving control data table includes data to be set tothe D/A conversion channels DAC1 and DAC2 for each use environmentaltemperature in order to obtain a predetermined target optical output andis formed in the flash memory 173 of the microcomputer 170.

FIG. 5 shows one example of the driving control data table TBL. Thedriving control data table TBL has an initial data region Eini and acorrection data region Ecor. The respective regions Eini and Ecor areassociated with each other every temperatures.

Initial data corresponding to an initial temperature characteristic ofthe LD100 is stored in the initial data region Eini. The initial data isdata for determining a bias current and a modulating current forobtaining the intended emission intensity or power every temperatures.For example, data about an initial bias current Ib(0), data about aninitial modulating current Imod(0) and data about their total current(driving current) Id(0) necessary to obtain a fixed emission power like0.8 mW are held in the initial data region for each predeterminedtemperature.

The relationship between the driving current (Id) and emission power(P0) of the LD100 is shown in FIG. 6, for example. Bias currents Ib(Ti),Ib(Tj) and Ib(Tk), threshold currents Ith(Ti), Ith(Tj) and Ith(Tk), andmodulating currents Imod(Ti), Imod(Tj) and Imod(Tk) attypically-illustrated temperatures Ti, Tj and Tk are shown in the samedrawing. Each bias current referred to above is set to a currentcorresponding to about 90% of its corresponding threshold current , forexample. As is apparent from FIG. 6, if the temperature rises whenattempt is made to obtain the fixed emission power, then both thethreshold and modulating currents must be made great according to itsrise. On the other hand, a temperature characteristic of the LD100 isshown in FIG. 7. Since the threshold current Ith and the driving currentId respectively show a characteristic non-linear with respect to thetemperature as is apparent from the same drawing, it is necessary tocontrol even the bias current Ib and the modulating current Imodnon-linearly with respect to the temperature (T) in a manner similar tothe above. Particularly, since the threshold current Ith increasessuddenly as the temperature rises, a control method for varying the biascurrent I simply linearly with respect to the temperature encountersdifficulties in most suitably setting the bias current Ib. Namely, whenthe temperature varies during the execution of the auto power control,the driving current of the LD100 increases or decreases so that theintensity of light becomes constant. However, the setting of the biascurrent cannot follow a variation in threshold current Ith and hence themodulating current Imod cannot be set to the optimum value either in amanner similar to the above. In the optical transceiver 1T, data aboutthe optimum bias current Ib(0) and modulating current Imod(0)corresponding to the temperature characteristics related to the initialthreshold current Ith and driving current Id of the LD100 are stored inthe initial data region Eini for each predetermined temperature. Thus,if the LD100 is driven using the data about the initial bias currentIb(0) and initial modulating current Imod(0) of the LD100, correspondingto the respective temperatures, then an emission delay and a quenchingfailure can substantially be solved.

The correction data region Ecor shown in FIG. 5 includes data about adifference bias current δIb and data about a difference driving currentδId as correction data for defining each current superimposed on a biascurrent and a modulating current defined by the initial data. Data abouta driving current Id(t) stored in the correction data region Ecor isdata which means a current obtained by superimposing a current definedbased on the data about the difference driving current δId on a drivingcurrent Id(0) defined based on the data about the initial bias currentIb(0) and the data about the initial modulating current Imod (0).Incidentally, while the driving control data shown in FIG. 5 areillustrated as data expressed in mA units for convenience, the drivingcontrol data are actually defined as digital data set every temperaturesfor defining such current values.

FIG. 8 shows the fore-and-aft relationship of life and degradationbetween a driving current Id and emission power Pf of the LD100 under afixed temperature environment. In FIG. 8, (0) shows one example of aninitial Id-Pf characteristic of the LD100, and (t) indicates one exampleof an Id-Pf characteristic of the LD100 after the elapse of apredetermined period. The characteristic indicated by (t) is acharacteristic deteriorated due to secular changes. Both the biascurrent and modulating current must be increased to obtain the sameemission power. When the characteristic changes from (0) to (t) due tothe secular changes, the whole driving current must be increased by δIdwith respect to the characteristic indicated by (0) to obtain the sameemission power. The driving current with respect to the initialcharacteristic indicated by (0) can be determined based on the initialdata. When the characteristic is deteriorated, the CPU170 controls thedriving current of the LD100 so that the bias current is increased byδIb and the modulating current is increased by δId−δIb. Data fordetermining increases in bias current and modulating current due to thedeterioration of the characteristic of the LD respectively correspond tothe data about the difference currents δIb and δId. When the LD100deteriorated in characteristic is driven using data at a temperature of73° C. shown in FIG. 5 when the temperature of the LD100 is 73° C., forexample, the bias current is determined based on the value of Ib(0)+δIband the modulating current is determined based on the value ofImod(0)+δId−δIb.

Data about currents Ib(0), Imod(0) and Id(0) for each predeterminedtemperature, each of which corresponds to the intended emission power,are initially stored in the initial data region Eini shown in FIG. 5.The data about those currents Ib(0), Imod(0) and Id(0) are a externallydown-loaded through the micon interface MCIF, for example. Thedown-loaded data can be written into the flash memory 173 under thecontrol of the CPU170. Alternatively, the data can also be writtentherein in a boot program mode in a microcomputer manufacturing process.

The correction data region Ecor shown in FIG. 5 is first initially setto a value like “0”. The CPU170 calculates correction driving controldata according to the degree of deterioration of the LD100 and stores ittherein. The once-stored correction driving control data is updated eachtime the deterioration of the LD increases or develops.

<<Driving control of LD>>

A procedure for generating data for driving control of the LD100 andcorrection driving control data therefor, using the driving control datatable will be explained based on FIG. 9.

After power-on has been reset (S1), the CPU170 detects the temperaturethrough a cooling process to be described later (S2). The temperaturedetection is carried out by obtaining data detected by the temperaturesensor 112 through the A/D conversion channel ADC4. The CPU170 readsinitial driving control data and correction driving control datacorresponding to the detected temperature from the driving control datatable TBL (S3). The read data are defined as the data about the initialbias current Ib(0), the data about the initial modulating currentImod(0), the data about the driving current Id(0) obtained by summing upthem, the data about the difference bias current δIb, the data about thedifference driving current δId and the data about the driving currentId(t). The CPU170 sets data of Ib(0)+δIb to the D/A conversion channelDAC1 based on the read data and controls a bias current to be suppliedto the LD100 through the transistor Tr1 (S4). Further, the CPU170 setsdata of Imod(O)+δId−δIb to the D/A conversion channel DAC2 and controlsa modulating current to be supplied to the LD100 through the transistorTr2 (S5). Since the correction data region Ecor of the driving controldata table TBL is filled with such a predetermined initializing codethat all the bits are “0”, δId=0 and δIb=0 when the optical transmissioncontrol device 1 is in first use. The emission intensity or power iscontrolled fixedly by starting bias current control and modulatingcurrent control and executing the auto power control by the APC111.

When a predetermined interval has elapsed, the CPU170 detects thetemperature in a manner similar to the above according to a timerinterrupt or the like (S6) and reads data about Id(0) and Id(t) in thecolumn corresponding to the detected temperature. If Id(t)=0, then theCPU170 recognizes Id(0) as a driving current corresponding to thedetected temperature. If Id(t)≠0, then the CPU170 recognizes Id(t) as adriving current corresponding to the detected temperature at that time(S7) Since the so-recognized driving currents (also called simplyregulated driving currents Idregu) are based on the temperature detectedin the temperature detection Step S6 different from Step S2, the drivingcurrents might be different from the driving currents set in Steps S4and S5 if an atmosphere temperature changes.

Next, the CPU170 obtains the values of voltages applied to the emittersof the transistors Tr1 and Tr2 through the A/D conversion channels ADC1and ADC2 to thereby measure a driving current subjected to auto powercontrol under the driving current control in Steps S4 and S5 andactually supplied to the LD100 (S8). The so-measured actual drivingcurrent is simply called monitor driving current Idmoni.

The CPU170 determines whether the measured monitor driving currentIdmoni exceeds a range X allowable for the regulated driving currentIdregu and increases (S9) The allowable range X is a range in which aquenching failure and an emission delay due to an increase in biascurrent under the auto power control substantially show no problem inthe regulated driving current Idregu at the temperature at that time,for example. This is equivalent to a current corresponding to a few % orso of the regulated driving current Iregu, for example. Namely, whetherthe monitor driving current Idmoni exceeds the range X allowable for theregulated driving current Idregu will be defined as a decision indexindicative of whether non-negligible deterioration in characteristicoccurs in LD100.

If the monitor driving current does not exceed the allowable range X,then the CPU170 determines that no non-negligible deterioration incharacteristic has occurred in the LD100 in relation to the temperaturedetected in Step S6 and is returned to the process of Step S2, where thetemperature detection is done again, and thereafter the driving currentcontrol of the LD100 is updated based on driving control data at thecorresponding temperature.

If it is detected that the monitor driving current has exceeded theallowable range X, then the CPU170 determines that the non-negligibledeterioration in characteristic has occurred in the LD100 in relation tothe temperature detected in Step S6 and performs a process of Step S10.First, the CPU170 sets data about the monitor driving current Idmoniobtained in Step S8 as data about a driving current Id(t) subsequent tothe deterioration of the characteristic and sets data about thedifference between the monitor driving current Idmoni and the regulateddriving current Idregu as data about a correcting difference drivingcurrent δId. Further, the CPU170 acquires data about a correctingdifference bias current δIb by the calculation of δId.α. Thereafter, theCPU170 writes the data about the driving current Id(t) subsequent to thecharacteristic deterioration, the data about the correcting differencedriving current δId and the data about the correcting difference biascurrent δIb into a correction data region Ecor in the columncorresponding to the temperature detected in Step S6. Thus, correctiondata corresponding to the non-negligible characteristic deteriorationdeveloped in the LD100 is set to the correction data region Ecor in thecolumn of the temperature detected in Step S6.

Thereafter, the routine procedure is returned to the process of Step S2where the temperature detection is done again and the driving currentcontrol of the LD100 is updated based on driving control data at thecorresponding temperature

Here, the operation expression of δIb=δId.α in Step S10 is one exampleapproximately determined according to experimental data. The value of αmay be determined in a range in which it does not exceed the thresholdcurrent of the LD100 subsequent to the characteristic deterioration, theemission delay is not excessively large and no undesired deformationoccurs in emission waveform. For example, it can be set as α=0.8.Namely, when the characteristic of the laser diode deteriorates, atendency shown in FIG. 10 could be obtained from experiments as a resultof discussions by the present inventors as to the relationship betweenan increase δId in driving current and an increase in δib in biascurrent due to the auto power control. It was revealed that this waswell fit to a tendency that if, based on it, the increase δId in themeasured driving current is set to a factor of a constant a smaller than1, the value obtained thereby is defined as an increase δIb in biascurrent, and the difference between the increase δId in the measureddriving current and the increase δIb in bias current is given as anincrease in modulating current, then an increase in driving current forobtaining fixed emission power would gradually increase according to thedevelopment of deterioration of the laser diode. Thus, when a method ofsetting the range allowable for the increase in driving current to thecurrent value corresponding to the few % or so and updating thecorrection data according to the degree of the development ofdeterioration in the laser diode as described above is adopted, thecalculation of the correction data becomes extremely easy and theaccuracy capable of enduring practical use can be ensured.

FIG. 11 shows the driving control's contents allowed for thecharacteristic deterioration of the LD according to the progress of thedeterioration of the LD. In FIG. 11, the temperature of the LD100 isfocused on a constant temperature to simplify a display format. Since nosubstantial characteristic deterioration occurs in the LD upon theinitial operation of the optical transmission device, the LD100 isdriven by the data about the initial bias current Ib(0) and the dataabout the initial modulating current Imod(0) (T1) . When the actualizeddeterioration of LD100 is detected according to the aboveIdmoni−Idregu>X (T2), correction data about a driving current Id(t), adifference current δId and a difference bias current δIb are registeredin their corresponding temperature column of the driving control datatable TBL in the above-described manner (T3). The LD100 under thetemperature at which the correction data have been registered, issubjected to bias current control by Ib(0) +δIb and subjected tomodulating current control by Imod(0)+δId−δIb (T4). When thedeterioration of the LD further proceeds, the deterioration of the LD100is detected according to the Idmoni−Idregu>X (T5). Idregu at this timeis determined in consideration of correction data in a manner similar tothe driving current control in T4. Thus, correction data about a drivingcurrent Id(t), a difference current δId, and a difference bias currentδIb, which are obtained in the next T6, are determined in considerationof the correction data which have been used so far. The so-obtained newcorrection data about the driving current Id(t), difference current δIdand difference bias current δIb are registered in their correspondingtemperature column of the driving control data table TBL so that thecorrection data are updated (T6). T4 through T6 are hereafter repeateduntil the deterioration of the LD100 reaches a specific limit value. Onsuch occasions, the correction data are updated. Incidentally, the limitvalue for the deterioration of the LD100 can be defined as when the biascurrent reaches twice the current value determined based on the dataabout Ib(0). The state thereof can be simply grasped by comparing dataabout each bias current sampled through the A/D conversion channel ADC1and the data about Ib(0).

According to the driving control method for the LD100, which has takeninto consideration the characteristic deterioration, the driving currentof the LD100 is controlled using the driving control data correspondingto the detected temperature. Upon determination of the degradation ofthe LD100, the ambient temperature of the LD100 is further measured anda decision is made as to whether the difference δId between a drivingcurrent defined by driving control data corresponding to the newlymeasured temperature and an actual driving current formed by the autopower control exceeds an allowable value. When the difference is foundto exceed the allowable value, it is determined that the deteriorationof the LD100 is advanced. Thus, a distinction, between whether theincrease in driving current due to the auto power control results fromthe deterioration of the LD100 and whether it results from a variationin ambient temperature is reliably made. The driving control datacorresponding to the corresponding temperature is updated based on thedifference δId between the driving currents used to determine thedeterioration of the LD100. After the renewal of the driving controldata, the driving current for the LD100 is controlled using the updateddriving control data. Thus, a quenching failure and an emission delayare limited to the minimum with respect to both the change in ambienttemperature and the characteristic deterioration of the LD100 due tosecular changes, whereby an optical output can be kept constant.

<<Relaxation of change in wavelength>>

The driving current of the LD100 increases as the characteristicdeterioration proceeds. An increase in driving current will cause a risein the temperature. It is known that in the laser diode, a variation inwavelength relative to a change in temperature of 1° C. is normally 0.07nm and the wavelength varies by about 0.1 nm each time the bias currentincreases by 30 mA. Wavelength-division multiplexing transmission andhigh-speed optical transmission need to strictly control the variationin wavelength. In order to meet such a demand, a cooling device 200 suchas a Peltier device, a cooling driver 201 and a D/A conversion channelDAC3 for supplying driving data to the cooling driver 201 can be addedto the optical transmission device shown in FIG. 3.

When the construction for relaxing the change in wavelength is added,the CPU170 performs the cooling process shown in FIG. 9. One detailedexample of the cooling process is shown in FIG. 12. In the coolingprocess, a temperature detecting process (S11), a process forrecognizing a regulated driving current Idregu (S12) and a process forobtaining a monitor driving current Idmoni (S13) similar to Steps S6, S7and S8 referred to above are carried out. It is determined whether thedifference between the regulated driving current Idregu and the monitordriving current Idmoni has exceeded a predetermined value Y (S14). If itis now desired to relax a change in wavelength of 0.1 nm or more, thenthe change in wavelength of 0.1 nm occurs when an increase in drivingcurrent is about 30 mA. Since it corresponds to a rise in thetemperature of about 1.4° C., of the LD100, the predetermined value Ycan be set to a value indicative of, for example, 15 mA or 30 mA or thelike. When the result of determination in Step S14 exceeds the value Y,cooling driving data for lowering the temperature of the LD100 by apredetermined temperature is loaded into the cooling driver 201 throughthe D/A conversion channel DAC3 to thereby activate the cooling device200. If the value Y is a value equivalent to 15 mA, for example, thencontrol for reducing the temperature of the LD100 by 0.7° C. is carriedout. If the value Y is a value equivalent to 30 mA, then control forreducing the temperature of the LD100 by 1.4° C. is performed.

With respect to the increase in driving current due to thecharacteristic deterioration of the LD100, the relaxation of thevariation in wavelength by cooling and the process for correcting thedriving control data are carried out independently of each other.

While the invention made by the present inventors as described above hasbeen described specifically by the illustrated embodiments, the presentinvention is not limited to the embodiments. Various changes can be madethereto within the scope not departing from the substance thereof.

For example, the emission output to be outputted from the opticaltransmission device 1 is one having the property of being physicallydetermined according to a communication environment under which it isplaced. For example, it can be notified to the CPU170 in accordance withan operating program of the CPU170, external instructions or a signaloutputted from a circuit like a dip switch. When emission powercorresponding to each driving control data held by a driving controldata table and instructed emission power are different from each other,the driving data in the driving control data table can be used by takinga factor of a ratio corresponding to a ratio between both data.

The initial driving control data can be generated by actually operatingthe optical transceiver 1T and based on the result of measurement ofeach current flowing in the laser diode. If such a generating method isused, it is then possible to obtain the initial driving control datawhich has taken into consideration the difference between thetemperature characteristic of the laser diode and the temperaturecharacteristic of the transistor of the LD driver.

The optical transmission device is not limited to the embodiment of theinterface board illustrated in FIG. 2. For example, the opticaltransceiver can take a form used as an optical transceiver module oroptical transmission module incorporated in a casing. The order in whichthe driving data setting process of the LD (S2 through S5 in FIG. 9),the data correcting process corresponding to the characteristicdeterioration of the LD (S6 through S10 in FIG. 10) and the coolingprocess are carried out, is not limited to that shown in FIG. 9.

INDUSTRIAL APPLICABILITY

The present invention can be widely applied to an optical transmissionsystem such as a PDS (Passive Doubler Star) or the like in which anoptical fiber is introduced into subscribers of telephones and ISDN, andan ATM-LAN (Asynchronous Transfer Mode-Local Area Network) or the like.

What is claimed is:
 1. A method of driving a laser diode suitable foruse in an optical transmission device including a current supply circuitfor supplying a bias current and a modulating current superimposed onthe bias current to a laser diode as driving currents, an automaticoptical output control circuit for supplementing the shortage of thedriving currents so that emission power of said laser diode is heldconstant, a temperature detection circuit for detecting an ambienttemperature of said laser diode, memory means storing initial drivingcontrol data for initially determining a modulating current and a biascurrent necessary to obtain predetermined emission power therein foreach predetermined temperature, and control means for driving andcontrolling said laser diode, comprising the following processes: afirst process for detecting an ambient temperature of said laser diodeby said temperature detection circuit; a second process for obtainingdriving control data corresponding to the temperature detected in saidfirst process from said memory means; a third process for controllingeach driving current to be supplied from said current supply circuit tosaid laser diode based on the driving control data obtained in saidsecond process; a fourth process for measuring each driving currentactually supplied to said laser diode whose emission power is heldconstant by said automatic optical output control circuit; a fifthprocess for detecting an ambient temperature of said laser diode by saidtemperature detection circuit; a sixth process for detecting whether thedifference between a driving current determined by driving control datacorresponding to the temperature detected in said fifth process and thedriving current measured in said fourth process exceeds an allowablerange; a seventh process for updating the driving control data relatedto the temperature, on said memory means so that the difference betweenthe driving currents is defined as each of increases in bias current andmodulating current when it is detected in said sixth process that saiddifference has exceeded the allowable range; an eighth process fordetecting an ambient temperature of said laser diode by said temperaturedetection circuit; a ninth process for obtaining updated driving controldata corresponding to the temperature detected in said eighth processfrom said memory means; and a tenth process for controlling each drivingcurrent to be supplied from said current supply circuit to said laserdiode based on the updated driving control data obtained in said ninthprocess.
 2. The method according to claim 1, wherein said drivingcontrol data comprises initial data for initially determining the biascurrent and the modulating current for each predetermined temperatureand correction data added to the initial data subsequently in saidseventh process, and said correction data is data for defining thedifference between the driving currents as each of the increases in biascurrent and modulating current.
 3. The method according to claim 2,wherein said seventh process includes a process for including data aboutthe difference between the driving currents and a value obtained byincreasing the difference between the driving currents by a factor of aconstant ratio smaller than 1 in said correction data as data about anincrease in bias current and, said tenth process determines a biascurrent by the sum of the initial bias current data included in theinitial data and the data about the increase in bias current included inthe correction data and adds the initial modulating current dataincluded in the initial data to the difference between the data aboutthe difference between the driving currents and the data about theincrease in bias current respectively included in said correction datato thereby determine a modulating current.